The invention relates to devices and methods for reducing surface friction drag on the hulls of shallow-draft watercraft, such as recreational boats and personal watercraft, and water sport devices, such as wakeboards and water skis. As used herein and in the claims, the term “watercraft” is intended to include all of the above-described watercraft and water sport devices.
Surface friction drag or “skin friction” drag is a significant component of the total power required to propel a watercraft through water. Reducing surface friction drag enables watercraft to travel at higher speeds and/or more efficiently. Accordingly, reducing surface friction drag has been the subject of a great deal of research in the field of watercraft design.
The magnitude of surface friction drag on the submerged surface of a watercraft depends, in part, upon the viscosity of the liquid through which the watercraft is traveling (usually fresh or salt water), the density of the liquid and the surface tension between the liquid and the submerged surface.
The effects of surface friction drag are focused in a “boundary layer,” a layer of liquid in which momentum is transferred from the submerged surface to the liquid. Momentum transfer is the greatest in the portion of the liquid that is closest to the submerged surface and decreases to the edge of the boundary layer. Momentum transfer in the boundary layer results in a reduction in the velocity of the water relative to the submerged surface, as well as turbulence.
One means of reducing surface friction drag is the introduction of a gas into the boundary layer, which reduces the fluid density and viscosity in the boundary layer. The relatively low density and viscosity of the gas results in less momentum transfer, and therefore, less surface friction drag. This technique is sometimes referred to in the art as “air lubrication”.
Air lubrication has been successfully implemented in hovercraft, in which the vessel sits atop a large cushion of air. Air cushions are not practical for use with most other types of watercraft, however, because water pressure increases with depth, which causes the air cushion to quickly rise to the surface of the water. Enormous amounts of power are required to push an air cushion down into even a few inches of water. This problem has been addressed, in part, by using small bubbles of air (i.e., micro-bubbles) instead of a larger air cushion. Small bubbles rise much more slowly in water than a large air cushion.
Full-scale use of micro-bubbles has been proven very difficult. The inventions of the prior art have faced three major technical challenges in successful use of micro-bubbles to reduce surface friction: (1) injecting micro-bubbles at a sufficient volumetric rate to fill a significant portion of the boundary layer, (2) keeping the micro-bubbles from migrating out of the boundary layer, and (3) adjusting the volumetric flow rate of micro-bubbles as the velocity of the watercraft changes.
Most prior art air lubrication systems use either a pump or pressurized air to supply the volume of micro-bubbles. This approach is deficient in several respects. Firstly, power must be expended to pump or pressurize the air. In all known prior art systems, the power expended to pump or pressurize the air completely offsets the power savings from reduced surface friction drag. Secondly, it is very difficult to inject pumped or pressurized air into the boundary layer. A typical boundary layer is only a few millimeters thick near the bow of the watercraft, which is where the air is injected in most prior art systems. Given that the micro-bubbles themselves are at least one millimeter in diameter and are typically injected at an angle to the direction of flow of the boundary layer, it is very difficult to prevent the micro-bubbles from passing through the boundary layer and into the free-flow water area. Thirdly, the prior art does not provide for an injection flow rate for micro-bubbles that varies in proportion to the watercraft's speed. This results in the micro-bubble injection rate being ideal at only one speed. At all other speeds, the injection rate is higher or lower than the ideal rate.
Pumped or pressured air systems are particularly unsuitable for recreational watercraft because they add significant weight and cost. In the case of wakeboards and water skis, these types of systems are completely impractical.
Accordingly, there is a need for an efficient, light-weight and inexpensive air lubrication system that is capable of entraining air bubbles into the boundary layer of a wide variety of shallow-draft watercraft.